Many forms of rubber, particularly synthetic rubber, such as styrene-butadiene (SBR) exhibit relatively low polarity compared with other rubbers. Unlike natural rubber for example, SBR does not develop surface peroxidal activity upon mastication, and as a result demonstrates poor inherent and/or processed tack.
A composition possesses "tack" if, under the condition of application, only light pressure produces a bond of sufficient strength between separate pieces of the material such that restoring the bond interface to its original separated state requires performing work on the bond. Tack is essential in the production of rubber, which requires strong tackifying action and good tack for processability. Additionally, the production of plyed rubber tire components such as tires, belts, hoses and other rubber parts comprising multiple layers of rubber depends upon rubber having good tack properties.
Adding tackifiers such as resins or natural rubber to compositions with poor tack properties, such as SBR, increases the tack of the resulting rubber compositions, and thereby increases the tendency of the composition to adhere to itself. However, adding natural rubber to impart tack properties to synthetic rubber defeats the very purpose of synthetic rubbers, designed to replace expensive natural rubber. Accordingly, the rubber industry has for the most part relied upon phenolic resins, such as modified alkylphenols of the type illustrated in FIG. 3 to impart tack to rubber compositions. Although these resins are effective in producing the desired tack, they are still relatively expensive and suffer from obvious environmental drawbacks. Hydrocarbon resins, such as tall oil-based resins, have also been used with limited success.
As a result of the disadvantages of the prior art tackifiers, the rubber industry has for years sought less costly, more environmentally acceptable substitutes for phenolic tackifier resins, with very limited success. For example, a tall-oil pitch and Kraft lignin coprecipitate has been developed for use as a rubber tackifier. See U.S. Pat. No. 4,287,104. The use of Kraft lignin in this application has not flourished commercially, however, in view of the need for expensive purification or derivatization of the Kraft lignin and the requirement of cumbersome Kraft lignin tall oil pitch coprecipitation procedures which are generally impractical to carry out in a rubber production facility.
The rubber industry therefore maintains a continuing need for an inexpensive, effective and environmentally agreeable alternative to presently available rubber tackifiers.
The pulp and paper industry produces tremendous quantities of Kraft lignin and lignosulfonate which, for the most part, are either burnt as fuel in high pressure boilers or discharged as waste with a consequent negative environmental impact. Although wood chemists have for many years addressed the problem of locating useful non-fuel applications for these wood lignins, currently less than 2% of all lignin available from spent pulping liquors are recovered and marketed for non-fuel uses in the United States. Accordingly, there exists a long-standing and ongoing need to implement new, non-fuel ways of effectively using lignin and other biomass by-products.
The use of materials of vegetable origin in rubber compounds has been reported in the past. These references generally refer to the proposed use of Kraft lignins as reinforcing agents, extenders, fillers and substitutes for carbon black. See, e.g., U.S. Pat. Nos. 3,312,643; 3,296,158; 3,282,871; 3,163,614; 3,991,022; 2,610,954; 3,364,158; 2,802,815; 3,984,362; and 3,817,974. See also Fries, "Development of Resins for the Rubber Industries" (1984), referring to the use of lignin only in reinforcing and dispergator applications. The Fries reference specifically mentions that products based on lignin have failed to gain importance in the rubber industry. This is due once again to the expensive purification/derivatization procedures required for Kraft lignin, discussed above.